It is already known to manufacture a sieve having at least one substantially plane sieve sheet by embedding the rim of the sieve sheet in a rib of plastics material. It is also known to provide stiffening ribs extending across the surface of the sieve sheet. However, such sieves are expensive in manufacture, and the stiffening ribs do not prevent warping of the sieve sheet.
It is also known to manufacture a sieve that is composed of a plurality of individual sieve sections which are joined by plastics ribs. Such planar sieve structures do not prevent warping, too (GB Pat. No. 741,918).